The invention relates to hydraulically actuated wheel brakes in which the brake pressure is transmittable by pistons slidable within wheel brake cylinders to brake shoes furnished with friction linings, which brake shoes are pressable against relevant friction surfaces of the wheel, for example against brake discs or brake drums, for a conversion into heat of the energy of motion of the vehicle.
In the known wheel brake systems of this kind, the pistons which slide the brake shoes and are guided within the wheel brake cylinders, are directly connected with the pertaining power chambers in the master cylinder by means of a hydraulic fluid, the so-called brake fluid. As a consequence, the brake fluid must have properties which are essential with regard to the efficiency of the braking system under the varied conditions encountered in general automotive vehicle use. Such conditions include both the specific conditions prevailing at the location of the master cylinder and to those in the wheel brake cylinder, in the connecting lines and in any additional units, such as brake pressure modulators, which may be included in the system. Considering that the safety of a vehicle depends on the condition of the braking system and, therefore, also on the physical behaviour of the brake fluid, these prerequisites lead to severe requirements with regard to the reliability of the brake fluid, that is to say, with regard to the long-term stability of the required properties, even in adverse circumstances.
In view of the fluid requirements both in the master cylinder and in the brake with respect to long-term stability, characteristics of the fluid regarding boiling point, behaviour toward water, compressibility, corrosion resistance, agressiveness toward the rubber sealing elements, viscosity over the entire temperature range and the lubricating power of a hydraulic fluid suitable to serve as a brake fluid must be specified and controlled. Attempts to meet these requirements sometimes lead to contradictory results. For instance, solubility in water would be favorable since any penetrating water causing corrosion would be rendered harmless, if, however the boiling point were not reduced to inadmissably low values due to the absorption of water. The hygroscopic behaviour of the customary brake fluids on the basis of glycol lead to the result that the brake fluids have to be changed at relatively short intervals. In this case due to the absorption of water, the boiling point decreases to such an extent that vapor locks occur in the liquid as an effect of the strong development of heat in the wheel brake, the vapor locks impairing the functioning of the brake to a dangerous degree due to the elevated compressibility of the vapor.
The boiling point of the currently used hygroscopic brake fluids is dependent on the water content and, therefore, is subject to aging. This feature gains more and more importance because cooling of the wheel brakes and of the brake fluid in the wheel brake is becoming more difficult for a number of reasons. For instance, the desire for aerodynamic vehicle bodies leads to designs which render it more difficult to convey cooling air supply to the wheel brakes. As compared with asbestos-containing brake linings, those made of materials free of asbestos, which are demanded increasingly have a considerably higher heat transmission which leads to increased heating of the brake fluid. These facts and still others have the result that elevated temperatures occur at least locally in some brake designs and may heat the brake fluid to the boiling point.